Item_func_user doesn't calculate anything in it's val_str() method,
just returns saved str_value.
Though Item::save_in_field method can destroy str_value, relying on
val_str() return. As a result we get the garbage stored in field.

We cannot use Item::save_in_field implementation for Item_func_user,
reimplement it in simpler way.


mysql-test/r/rpl_session_var.result:
  Bug #29878 Garbage data generation when executing SESSION_USER() on a slave.
  
  test result
mysql-test/t/rpl_session_var.test:
  Bug #29878 Garbage data generation when executing SESSION_USER() on a slave.
  
  test case
sql/item.cc:
  Bug #29878 Garbage data generation when executing SESSION_USER() on a slave.
  
  duplicating code moved to Item::save_str_in_field
sql/item.h:
  Bug #29878 Garbage data generation when executing SESSION_USER() on a slave.
  
  duplicating code moved to Item::save_str_in_field
sql/item_strfunc.h:
  Bug #29878 Garbage data generation when executing SESSION_USER() on a slave.
  
  Item_func_user::save_in_field implemented as simple storing str_value